The separation of mixtures of gases, especially air, by so-called pressure swing adsorption processes is well-known to those skilled in gas separation technology. In principle, a PSA process involves the following steps:
(a) contacting a gaseous feed mixture of two or more different gases with an adsorbent under a first, relatively high, pressure, thereby causing at least one of the gases to be selectively adsorbed into the adsorbent, and producing a gaseous first product mixture containing a greater proportion of the less selectively adsorbed gas or gases than the feed mixture;
(b) separating the first product mixture from the adsorbent; and
(c) reducing the pressure on the adsorbent to a second pressure lower than the first pressure, thereby causing desorption from the adsorbent sieve of a gaseous second product mixture containing a greater proportion of the selectively adsorbed gas(es) than the feed mixture.
In industrial practice, PSA processes tend to be considerably more complicated than this simple description would suggest, and usually involve several adsorption vessels (usually referred to as "beds"). For example, U.S. Pat. No. 3,430,418 to Wagner and U.S. Pat. No. 3,986,849 to Fuderer et al. describe PSA processes using multi-bed systems. Such cycles are commonly based on the release of void space gas from the product end of each bed in one or more cocurrent depressurization steps upon completion of the adsorption step. In these cycles, a portion of the released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and/or purged to desorb the more selectively adsorbed component of the gas mixture from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
U.S. Pat. No. 3,738,087 to McCombs describes a three-bed system for the separation and recovery of air and other gases. In one process described in this patent, air is added to an adsorbent bed for the repressurization of that bed. Nitrogen is then selectively adsorbed and oxygen is discharged from the product end of the bed at rates such that the bed pressure increases to the upper adsorption pressure. A PSA cycle incorporating the increasing pressure adsorption step includes (1) the increasing pressure adsorption step; (2) cocurrent depressurization to an intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower desorption pressure; (4) purge; and (5) partial repressurization. The void space gas released during the cocurrent repressurization step is employed in this process for (1) passage to other beds in the system in a pressure equalization step; (2) providing purge gas; (3) providing a pressure equalization sequence; and (4) to produce the product. The PSA cycle incorporating the increasing pressure obviates the constant pressure adsorption step employed in the Wagner cycle. Additionally, more time for bed regeneration is available during the steps of countercurrent depressurization and purge within a given cycle time. Greater productivity and recovery and/or purity are obtained with this improvement to a given system. This is particularly true in systems designed for relatively short overall cycle time operation.
U.S. Pat. No. 4,589,888 to Hiscock et al. describes a reduced cycle time, four-bed process. The gas released upon cocurrent depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Cocurrent depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
A PSA process suitable for the recovery of both less and more readily adsorbable components is described in British Patent 1,536,995 to Benkmann. This process involves two beds in a series cycle. The feed is introduced into the lower bed which retains the more readily adsorbable component. The feed step is followed by a copurge step in which the less readily adsorbable or "light" component is displaced in the lower bed by a recycled stream of "heavy" components, so that the lower bed at the end of the step contains only the heavy component. At this moment, the connection between the upper and lower beds is interrupted by an automatic valve and the heavy product is recovered from the lower bed by countercurrent depressurization. The upper bed is, in the meantime, also depressurized and purged to remove all the heavy components.
It is also known to use permeable membranes in conjunction with PSA systems. U.S. Pat. No. 4,398,926 to Doshi describes a PSA system in which a feed gas stream is passed over a permeable membrane to achieve bulk separation of impurities from the high pressure feed gas stream, with passage of the product-rich permeate gas to the PSA system at reduced pressure for final purification. To increase product recovery in the PSA system, a portion of the non-permeate gas from the permeable membrane is depressurized to the permeate pressure level, such as the adsorption pressure level, and is passed under pressure to the PSA system as a co-feed gas. The gas is passed to each adsorbent bed of the system in turn prior to commencing cocurrent depressurization.
The terms "pressure swing adsorption process" and "PSA process" as used herein include all the foregoing and similar processes for separating gaseous mixtures.
Whatever the exact details of the apparatus and process steps used in a PSA process, critical factors include the capacity of the adsorbent for the more adsorbable gas, the selectivity of the adsorbent, and the stability of the adsorbent. The selectivity of the adsorbent is normally measured in terms of separation factor (SF) which is defined by: EQU SF.sub.(x/y) =(A.sub.x *G.sub.y)/(A.sub.y *G.sub.x)
where A.sub.x and A.sub.6 are the concentration of adsorbates x and y in the adsorbed phase, and G.sub.x and G.sub.y are the concentrations of x and y in the gaseous phase.
In many PSA processes, zeolites are the preferred adsorbents because of their high adsorption capacity and their high selectivity; both these properties are due to the microporous nature of the zeolite structure, in which a large number of pores of a consistent size extend throughout the lattice framework and bare cations occupy well defined and consistent positions in the pores. For example, U.S. Pat. No. 2,882,243 to Milton describes the use of zeolite A having a silica/alumina ratio of 1.85.+-.0.5 and containing hydrogen, ammonium, alkali metal, alkaline earth metal or transition metal cations as an adsorbent for separating nitrogen and oxygen. U.S. Pat. No. 2,882,244 also to Milton describes a similar process but in which X is a different type of zeolite having a silica/alumina ratio of 2.5.+-.0.5.
U.S. Pat. No. 4,453,952 to Izumi describes the use of sodium zeolite A containing iron (II), and optionally potassium, as an adsorbent for separating nitrogen and oxygen.
European Patent Application No. 84730031.6 (Publication No. 122 874) describes the use of sodium faujasite as an adsorbent for separating nitrogen and oxygen.
U.S. Pat. Nos. 3,140,932 and 3,140,933, both to McKee, describe the use of X zeolites which have been ion-exchanged with alkali metal or alkaline earth metals as adsorbents for separating nitrogen and oxygen. Since the ion-exchange was not exhaustive and the X zeolites were synthesized using sodium as the templating agent, the partially ion-exchanged materials used are in fact mixed sodium/alkali metal or /alkaline earth metal zeolites.
Among all the molecules which can be adsorbed by A and X zeolites, water has the highest affinity. In order to secure optimum adsorption performance it is necessary to activate the zeolite by heating it to high temperatures in order to drive off as much adsorbed wateras possible; adsorbed water, even in small amounts, seriously diminishes the adsorption capacity of the zeolite. A portion of the adsorbed water in zeolites is held tenaciously, and typically, in industrial plants, the zeolite must be heated to 600.degree. to 700.degree. C. to drive off most of this tenaciously-held water. Unfortunately, some zeolites which have desirable adsorption properties are not stable to the high temperatures required to drive off this water. In particular, lithium A and X zeolites have a high adsorption capacity for nitrogen and good selectivity in nitrogen/oxygen separations, and may be used with good results in nitrogen/oxygen separations under industrial conditions; the use of lithium X zeolites in such separations is described and claimed in U.S. patent application Ser. No. 067,820, filed June 30, 1987 now U.S. Pat. No. 4,859,217. The thermal stability of lithium A and X zeolites is sufficient to permit their activation provided that the activation temperature is carefully controlled. However, under industrial conditions, where it may be necessary to activate several tons of adsorbent at one time, it is difficult and expensive to achieve precise control and uniformity of the activation temperature throughout the large mass of adsorbent being activated, so that it is desirable to use an adsorbent with a wide margin of safety between the temperature necessary for activation and the temperature at which damage to the crystal structure of the adsorbent occurs rapidly. In the case of lithium A and X zeolites, the margin of safety is somewhat less than is desirable for use under industrial conditions; for example, a lithium X zeolite may suffer rapid crystal collapse at approximately 740.degree. C., a temperature which does not leave a very wide safety margin if activation at 650.degree.-700.degree. C. is required.
It has now been discovered that lithium/alkaline earth metal A and X zeolites in which the lithium/alkaline earth metal molar ratios fall within certain ranges have thermal stabilities greater than the corresponding pure lithium zeolites and adsorption capacities and selectivities which are almost as good as those of the corresponding pure lithium zeolites. Accordingly, these lithium/alkaline earth metal A and X zeolites are useful in separating nitrogen and oxygen.